1. Field of the Invention
The present invention relates generally to a hydraulic control system for an automatic transmission of an automotive vehicle. More particularly, the invention relates to a fail-safe hydraulic control system which serves to establish a reverse mode securely even when failures occur in frictional elements of an automatic transmission and/or a control circuit.
2. Description of the Prior Art
Japanese Patent First Publication No. 2-120556 discloses an electrically controlled automatic transmission for an automotive vehicle. This transmission includes generally a plurality of frictional elements such as clutches and brakes which are actuated by a hydraulic control system. The hydraulic control system includes solenoid operated pressure control valves which are operable to modify pressure provided from a hydraulic power source to actuate the frictional elements respectively according to a given schedule for establishing preselected gear rations in forward and reverse modes.
However, such a prior art hydraulic control system for an automatic transmission encounters a drawback in that when frictional elements for establishing a reverse mode fail due to breaking of a wire in a control circuit or the so-called spool sticking in solenoid valves, sufficient pressures are not obtained for actuating the frictional elements for moving a vehicle backward even when a driver shifts a selector lever to a reverse position.